Unexpectedly, Applicants have invented ways to improve the performance of solid oxide cells at lower temperatures with certain embodiments of the present invention. In certain other embodiments, Applicants have achieved heretofore unknown room-temperature operation of solid oxide cells. Lower temperature and room-temperature operation of solid oxide cells dramatically expand the universe of possible applications of these versatile cells. Also, the array of materials now available to construct these cells has exploded, since exotic and expensive high-temperature materials are no longer required.
Solid oxide fuel cells, otherwise known as ceramic fuel cells, present an environmentally friendly alternative to mainstream electrical energy production processes involving the combustion of fossil fuels. Solid oxide fuel cells enable the catalytic conversion of chemical energy stored in hydrogen into electrical energy without the concomitant release of greenhouse gases. The generation of electrical current by a solid oxide fuel cell using a hydrogen fuel results in the production of water as opposed to the production carbon dioxide, nitrous oxides, and/or sulfur dioxides associated with the combustion of fossil fuels.
In addition to hydrogen, solid oxide fuel cells are operable to function on a wide variety of fuel sources. Fuel sources in addition to hydrogen include hydrocarbons such as methane, natural gas, and diesel fuel. Hydrocarbon fuel sources are reformed into hydrogen for use with solid oxide fuel cells. Hydrocarbon reforming can be administered prior to entry into the fuel electrode or can be administered at the fuel electrode of a solid oxide fuel cell. The ability to function on a wide variety of fuels distinguishes solid oxide fuel cells from other fuel cells which lack the ability to operate on various fuels. Furthermore, the ability of solid oxide fuel cells to administer hydrocarbon feedstock reformation frees such fuel cells from the limitations associated with hydrogen production and distribution.
Currently, solid oxide fuel cells operate at high temperatures ranging from about 800° C. to 1000° C. As a result of high operating temperatures, solid oxide fuel cells require the use of exotic materials which can withstand such operating temperatures. The need for exotic materials greatly increases the costs of solid oxide fuel cells, making their use in certain applications cost-prohibitive. High operating temperatures exacerbate stresses caused by differences in coefficients of thermal expansion between components of a solid oxide fuel cell. If the operating temperature could be lowered, numerous advantages could be realized. First, less expensive materials and production methods could be employed. Second, the lower operating temperature would allow greater use of the technology. Third, energy needed to heat and operate the fuel cell would be lower, increasing the overall energy efficiency. Fourth, a lower operating temperature increases the service life of the cell. Significantly, the high operating temperature is required because of poor low temperature ion conductivity.
Proton exchange membrane (“PEM”) fuel cells enjoy operational temperatures in the range 50-220° C. Typically relying on special polymer membranes to provide the electrolyte, PEM cells transmit protons across the electrolyte, rather than oxygen ions as in solid oxide fuel cells. However, high proton conductivity requires precise control of hydration in the electrolyte. If the electrolyte becomes too dry, proton conductivity and cell voltage drop. If the electrolyte becomes too wet, the cell becomes flooded. Electro-osmotic drag complicates hydration control: protons migrating across the electrolyte “drag” water molecules along, potentially causing dramatic differences in hydration across the electrolyte that inhibit cell operation. As a result, PEM cells are perhaps 40% less efficient than solid oxide fuel cells. Accordingly, it would be advantageous to obtain the low operating temperatures of the PEM fuel cell without the need to maintain strict control over electrolyte hydration, and to avoid the inefficiencies of those cells.
In certain circumstances, a solid oxide fuel cell can operate “in reverse” to electrolyze water into hydrogen gas and oxygen gas by inputting electrical energy. In other circumstances, a solid oxide electrolyzer cell can be designed primarily for use as a hydrolyzer, generating hydrogen and oxygen for later use. In still other circumstances, an electrolyzer cell can be used for other purposes, such as extraction of metal from ore and electroplating. In conventional electrolyzers, electrical energy is lost in the electrolysis reaction driving the diffusion of ions through the electrolyte and across the distance between the electrodes. Also, the ability to conduct electrolysis at higher temperatures would improve the efficiency of the electrolysis. However, at higher temperatures, electrolyzers face similar thermal stresses and cracking caused by differences in coefficients of thermal expansion between components of the solid oxide electrolyzer cell. Accordingly, better matching of coefficients of thermal expansion and lower operating temperatures are desired for electrolyzer cells.
A lambda sensor is a device typically placed in the exhaust stream of an internal combustion engine to measure the concentration of oxygen. That measurement allows regulation of the richness or leanness of the fuel/air mixture flowing into the engine. If the fuel/air stream contains too much oxygen, the quantity λ is greater than 1, and the mixture is too lean. If the fuel/air stream contains too little oxygen, then λ<1 and the mixture is too rich. λ equals 1, the ideal situation, when the mixture contains a stoichiometrically equivalent concentration of oxygen and hydrocarbon to allow for complete combustion. A lambda sensor positioned in the exhaust stream detects the amount of oxygen in the combustion products, thereby providing feedback regarding richness or leanness. Lambda sensors and other sensors rely on the diffusion of oxygen anions (O2−) and other ions through barrier materials in ways similar to the manner in which oxygen anions diffuse through a solid electrolyte of a solid oxide fuel cell. Moreover, given the high operating temperature of lambda sensors and similar devices, sensors face thermal stresses, cracking, and delamination issues similar to those facing fuel cells and electrolyzers. Accordingly, embodiments of the present invention provide for improved sensor technology by addressing ionic conductivity and mismatching of coefficients of thermal expansion, among other reasons.
It has recently been reported that adjacent atomically flat layers of strontium titanate (STO) with yttria-stabilized zirconia (YSZ) produce an interface that has a dramatically higher ionic conductivity for oxygen anions. J. Garcia-Barriocanal et al., “Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures,” 321 SCIENCE 676 (2008). Those authors concluded that growing thin epitaxial layers of YSZ on epitaxial STO caused the YSZ to conform under strain to the crystal structure of the STO, thereby creating voids in the YSZ crystal structure at the interface between the two materials. Those voids allowed an increase of oxygen ionic conductivity of approximately eight orders of magnitude relative to bulk YSZ at 500 K (227° C.). However, epitaxially-grown STO and YSZ require an extraordinarily clean environment and a relatively small scale, in addition to expensive deposition equipment. Furthermore, the geometries of establishing ionic communication between an electrode and an interface present another obstacle: the region for harvesting ions at the intersection of three materials (electrode, STO, and YSZ, for example) is by definition small compared to the contact area possible between an electrode and an electrolyte.
In view of the foregoing problems and disadvantages associated with the high operating temperatures of solid oxide cells, it would be desirable to provide solid oxide cells that can demonstrate lower operating temperatures. In addition, providing solid oxide cells and components that better tolerate higher temperatures would be advantageous. Moreover, the efficiency losses due to the thickness of electrolytes make thinner electrolytes desirable. Furthermore, it is also desirable to construct metal oxide electrolytes having dramatically higher ionic conductivities. Large-scale production of metal oxide electrolytes would be facilitated if higher ionic conductivities could be achieved without requiring epitaxial growth of electrolyte materials. It would be advantageous, also, if the geometry of harvesting ions at the intersection of electrode and electrolyte materials could be addressed.